1. Field of Invention
The present invention relates to a current detection circuit and a voltage converter using the current detection circuit, more particularly, to a circuit for use in mobile appliances and for detecting the output current of a power source circuit formed of a semiconductor integrated circuit.
2. Description of Related Art
In recent years, a power source circuit for use in various kinds of electronic appliances and for supplying stable DC power to a load is provided with a current detection circuit for mainly detecting the output current of the power source circuit.
For example, U.S. Pat. No. 4,885,477 discloses such a current detection circuit as shown in FIG. 4. In FIG. 4, an output current Iop is supplied from an input terminal InP through a main transistor 1p formed of an N-channel MOS transistor to a load 2P and an output terminal Outp, and a current proportional to the output current Iop flows to a control circuit 8p. Like the main transistor Ip, an auxiliary transistor 3p is formed of an N-channel MOS transistor. The drain terminal of the auxiliary transistor 3p is connected to the input terminal Inp and the gate terminal of the auxiliary transistor 3p is connected to a terminal VgP, as in the main transistor 1p. An adjustment circuit 4p is formed of an amplifier 40p and a P-channel MOS transistor 41p. The non-inverting input terminal of the amplifier 40p is connected to the source terminal of the main transistor 1p and the inverting input terminal of the amplifier 40p is connected to the source terminal of the auxiliary transistor 3p. The output of the amplifier 40p is connected to the gate terminal of the transistor 41p. The source terminal of the transistor 41p is connected to the source terminal of the auxiliary transistor 3p, and the drain terminal of the transistor 41p is used to output a detection current Isp to the control circuit 8p. 
With the above-mentioned configuration, the main transistor 1p supplies the output current Iop from the input terminal Inp to the load 2p through the output terminal Outp on the basis of the voltage applied to the terminal Vgp. Simultaneously, the auxiliary transistor 3p supplies the detection current Isp to the control circuit 8p through the transistor 41p on the basis of the voltage applied to the same terminal Vgp as that described above. At this time, the amplifier 40p performs feedback operation in which the gate terminal of the transistor 41p is controlled to adjust the impedance of the transistor 41p so that the source potential of the main transistor 1p becomes equal to the source potential of the auxiliary transistor 3p. With this feedback operation, the terminal potentials of the main transistor 1p become equal to the terminal potentials of the auxiliary transistor 3p with one another. The ratio of the drain-source resistance (ON resistance) of the auxiliary transistor 3p to that of the main transistor 1p is accurately inversely proportional to the ratio of the size of the auxiliary transistor 3p to that of the main transistor 1p. The ratio of the current flowing through the auxiliary transistor 3p to that flowing through the main transistor 1p is proportional to the ratio of the size of the auxiliary transistor 3p to that of the main transistor 1p. In other words, in the case that the size of the main transistor 1p is N times the size of the auxiliary transistor 3p, the detection current Isp flowing through the control circuit 8 is 1/N times the output current Iop (Isp=Iop/N).